Low energy ion gun having multiple multi-aperture electrode grids with specific spacing requirements

ABSTRACT

A low energy ion gun for ion beam processing. The ion gun includes a plasma chamber having an open ended, conductive, non-magnetic body, a first end of which is closed by a flat or minimally dished dielectric member and with electrodes at a second end thereof opposite the first end. The ion gun also has primary magnets arranged around the body for trapping electrons adjacent the wall of the plasma chamber in use of the ion gun and an r.f. induction device. The electrodes include multi-aperture grids arranged for connection to respective positive potential sources and positioned to contact the plasma in the plasma chamber. The apertures of the grids are aligned so that particles emerging from an aperture of a first one of the grids are accelerated through corresponding apertures of the other grids in the form of a beamlet. A plurality of beamlets forms a beam.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for generating a beam of chargedparticles, particularly to an ion gun for use in an ion beam processingapparatus and to an ion beam processing apparatus incorporating same.

Ion beams have been used for many years in the production of componentsin the micro-electronics industry and magnetic thin film devices in thestorage media industry. Typically, an ion beam, such as an argon ionbeam, is required to have a large area, a high current and an energy ofbetween 100 eV and 2 keV. The beam can be used in a number of ways tomodify the surface of a substrate, for example by sputter deposition,sputter etching, milling, or implantation.

In a typical ion beam source (or ion gun) a plasma is produced byadmitting a gas or vapour to a low pressure discharge chamber containinga heated cathode and an anode which serves to remove electrons from theplasma and to give a surplus of positively charged ions which passthrough a screen grid or grids into a target chamber which is pumped toa lower pressure than the discharge chamber. Ions are formed in thedischarge chamber by electron impact ionisation and move within the bodyof the ion gun by random thermal motion. The plasma will thus exhibitpositive plasma potential which is higher than the potential of anysurface with which it comes into contact. Various arrangements of gridscan be used, the potentials of which are individually controlled. In amultigrid system the first grid encountered by the ions is usuallypositively biased whilst the second grid is negatively biased. A furthergrid may be used to decelerate the ions emerging from the ion source soas to provide a collimated beam of ions having more or less uniformenergy. For ion sputtering a target is placed in the target chamberwhere this can be struck by the beam of ions, usually at an obliqueangle, and the substrate on to which material is to be sputtered isplaced in a position where sputtered material can impinge on it. Whensputter etching, milling or implantation is to be practised thesubstrate is placed in the path of the ion beam.

Hence, in a typical ion gun an ion arriving at a multiapertureextraction grid assembly first meets a positively biased grid.Associated with the grid is a plasma sheath. Across this sheath isdropped the potential difference between the plasma and the grid. Thisaccelerating potential will attract ions in the sheath region to thefirst grid. Any ion moving through an aperture in this first grid, andentering the space between the first, positively biased grid, and thesecond, negatively biased, grid is strongly accelerated in an intenseelectrical field. As the ion passes through the aperture in the secondgrid and is in flight to the earthed target it is moving through adecelerating field. The ion then arrives at an earthed target with anenergy equal to the potential of the first, positive, grid plus thesheath potential.

Hence, a conventional ion gun comprises a source of charged particleswhich are accelerated through an externally applied electric fieldcreated between a pair of grids. Conventionally, for low energy ion beamproduction, three grids are used, the first being held at a positivepotential, the second being held at a negative potential adjusted togive the best divergence, and the third, if present, at earth potential,i.e. the potential of the chamber in which the beam is produced. Beamsof this nature are well described in the open literature going back over25 years.

In some applications it is desirable to obtain an ion beam of maximumcurrent. However, in other applications it is the divergence of the ionscomprising the beam, or rather the relative lack of it, that is criticalto achieving a suitable process performance.

U.S. Pat. No. 4,447,773 discloses an ion beam accelerator system forextracting and accelerating ions from a source. The system includes apair of spaced, parallel extraction grids, 60 mil (1.524 mm) apart,having aligned pairs of holes for extracting ion beamlets. The pairs ofholes are positioned so that the beamlets converge and the mergedbeamlets are accelerated by an accelerator electrode which is 0.6 inch(15.24 mm) downstream of the extraction grid pair. The extraction gridsare formed with numerous small holes through which beamlets of ions canpass and are maintained a potential difference of a few hundred volts.The accelerator electrode has a single hole, which is slightly greaterin height than the height of the matrix of holes in the extractiongrids, and is maintained at a much lower potential for accelerating theconverged ion beam emerging from the extraction grid pair.

An extensive introduction to and prior art review of ion beam technologyis provided in EP-B-0462165. EP-B-0462165 itself describes an ion gun inwhich the plasma from which the ions are accelerated by the acceleratorgrid is at a low potential of not more than about 500 V and is ofuniform density so as to permit high current densities of the order 2 to5 mA/cm² in the ion beam to be achieved at low potential (i.e. less thanabout 500 V), and with minimum risk of damage to the accelerator grid,in operation. This system provides an ion gun in which the plasma can beefficiently generated using a commercially acceptable high radiofrequency, such as 13.56 MHz or a multiple thereof, and in which theresulting plasma has the desirable properties of high density, gooduniformity and a relatively low plasma potential.

However, a perennial problem with the ion beam sources described in theprior art is the high magnitude of divergence to which the beam issusceptible. Whilst the system of EP-B-0462165 solves many of theproblems associated with other prior art ion beam sources, it wouldstill be highly desirable to improve this system to provide a reduceddegree of ion beam divergence. There is a growing demand for ultra lowdivergence at low to medium beam energy. In this role the beam istypically described as an ion mill selectively etching deep trenches ofperhaps 1×10 μm scale length. To do so requires a beam with a divergenceof no more than approximately 1° at an energy dictated by theconstraints of maximum rate at a processing energy of perhaps as low as500 eV. Conventional ion guns operating at this energy cannot meet thedivergence requirement at a high enough current to meet the processrate.

SUMMARY OF THE INVENTION

The present invention accordingly seeks to provide an ion gun which iscapable of operation in a manner such that the above aims aresubstantially achieved.

According to the present invention there is provided apparatus for theproduction of low energy charged particle beams comprising: a plasmachamber; means for generating in the plasma chamber a plasma comprisingparticles of a first polarity and oppositely charged particles of asecond polarity; means for restraining particles of the first polarityin the plasma chamber; a first multi-aperture electrode grid contactingthe plasma, wherein the first electrode grid is arranged for connectionto a first potential source so as to impart to the first electrode grida first potential of the second polarity; a second multi-apertureelectrode grid arranged for connection to a second potential source soas to impart to the second electrode grid a second potential, the secondpotential being less than in the sense of being either less positivethan or less negative than the first potential so as to produce betweenthe first and second electrode grids a first acceleration field foraccelerating charged particles of the second polarity towards andthrough the second grid; and a third multi-aperture electrode gridarranged for connection to a third potential source so as to impart tothe third electrode grid a third potential of the first polarity and toproduce between the second and third electrode grids a secondacceleration field for accelerating charged particles of the secondpolarity towards and through the third electrode grid, the grid spacingbetween the first and second grids being greater at the periphery of thegrids than at the centre thereof, the apertures of the first, second andthird grids being aligned so that particles emerging from an aperture ofthe first grid are accelerated through a corresponding aperture of thesecond grid and then through a corresponding aperture of the third gridin the form of a beamlet, a plurality of beamlets from the third gridforming a beam downstream of the third grid.

The apparatus of the invention may be used to generate an electron beam.in which case the charged particles of the first polarity are ions andthe charged particles of the second polarity are electrons, or togenerate an ion beam, in which case the charged particles of the firstpolarity are electrons and the charged particles of the second polarityare ions.

Accordingly, the invention provides a low energy ion gun for use in ionbeam processing comprising: a plasma chamber comprising an open ended,conductive, non-magnetic body, a first end of which is closed by a flator minimally dished dielectric member and with electrodes at a secondend thereof opposite the first end; primary magnet means arranged aroundthe body for trapping electrons adjacent the wall of the plasma chamberin use of the ion gun; and an r.f. induction device including asubstantially flat coil which lies adjacent to the dielectric member forinductively generating a plasma in the plasma chamber, characterised inthat the electrodes include a first multi-aperture grid arranged forconnection to a first positive potential source and positioned tocontact the plasma in the plasma chamber; a second multi-aperture gridarranged for connection to a second potential source of lower potentialthan the first source so as to produce a first acceleration field foraccelerating ions towards and through the second grid; and a thirdmulti-aperture grid arranged for connection to a third potential sourceof lower potential than the second potential source so as to produce asecond acceleration field for accelerating ions towards and through thethird grid, the grid spacing between the first and second grids beinggreater at the periphery of the grids than at the centre thereof, theapertures of the first, second and third grids being aligned so thatparticles emerging from an aperture of the first grid are acceleratedthrough a corresponding aperture of the second grid and then through acorresponding aperture of the third grid in the form of a beamlet, aplurality of beamlets from the third grid forming a beam downstream ofthe third grid.

The provision of a grid spacing between the first and second grids whichis greater at the periphery of the grids than at the centre thereof isan important feature of these embodiments of the invention. Preferablythis variation in grid spacing is achieved by contouring one or both ofthe neighbouring surfaces of the first and second grids. Thus, in onepreferred embodiment, the second grid has a generally flat surfacetowards its periphery but in its central region bulges outwardly towardsthe first grid. The provision of this variation in grid spacing over thegrids recognises that the plasma density of the beam approaching thefirst grid tends to diminish towards the periphery of the beam. Theacceleration field to which individual beamlets are subject on passingthrough the first grid depends to some extent upon the grid spacing,which may therefore be selected to optimise the divergence of individualbeamlets, whether from the periphery or the central region of the firstgrid.

In one embodiment of the invention, the third potential source may bearranged to impart a negative potential to the third grid.Alternatively, the third potential source may be arranged to earth thethird grid. In this case, a fourth grid may be provided and arranged forconnection to earth.

The invention further provides a low energy ion gun for use in ion beamprocessing comprising: a plasma chamber comprising an open ended,conductive, non-magnetic body, a first end of which is closed by a flator minimally dished dielectric member and with electrodes at a secondend thereof opposite the first end; primary magnet means arranged aroundthe body for trapping electrons adjacent the wall of the plasma chamberin use of the ion gun; and an r.f. induction device including asubstantially flat coil which lies adjacent to the dielectric member forinductively generating a plasma in the plasma chamber, characterised inthat the electrodes include a first multi-aperture grid arranged for aconnection to a first positive potential source and positioned tocontact the plasma in the plasma chamber; a second multi-aperture gridarranged for connection to a second potential source of lower potentialthan the first source so as to produce a first acceleration field foraccelerating ions towards and through the second grid; a thirdmulti-aperture grid arranged for connection to a negative potentialsource so as to produce a second acceleration field for acceleratingions towards and through the third grid; and a fourth multi-aperturegrid arranged for connection to earth, the apertures of the first,second, third and fourth grids being aligned so that ions emerging froman aperture of the first grid are accelerated through a correspondingaperture of the second grid and then through a corresponding aperture ofthe third grid before passing through a corresponding aperture of thefourth grid in the form of a beamlet, a plurality of beamlets from thefourth grid forming a beam downstream of the fourth grid.

Also provided in accordance with the invention is a low energy electrongun for use in electron beam processing comprising: a plasma chambercomprising an open ended, conductive, non-magnetic body, a first end ofwhich is closed by a flat or minimally dished dielectric member and withelectrodes at a second end thereof opposite the first end; primarymagnet means arranged around the body for trapping ions adjacent thewall of the plasma chamber in use of the electron ion gun; and an r.f.induction device including a substantially flat coil which lies adjacentto the dielectric member for inductively generating a plasma in theplasma chamber, characterised in that the electrodes include a firstmulti-aperture grid arranged for connection to a first negativepotential source and positioned to contact the plasma in the plasmachamber; a second multi-aperture grid arranged for connection to asecond negative potential source of less negative potential than thefirst source so as to produce a first acceleration field foraccelerating electrons towards and through the second grid; and a thirdmultiaperture grid arranged for connection to a third potential sourceof higher potential than the second potential source so as to produce asecond acceleration field for accelerating electrons towards and throughthe third grid, the apertures of the first, second and third grids beingaligned so that electrons emerging from an aperture of the first gridare accelerated through a corresponding aperture of the second grid andthen through a corresponding aperture of the third grid in the form of abeamlets, a plurality of beamlets from the third grid forming a beamdownstream of the third grid.

The invention also provides a method for generating a low energy ionbeam, which method comprises;

(a) providing an ion gun according to the foregoing description;

(b) supplying to the plasma chamber a plasma forming gas;

(c) exciting the R.f. induction device to generate a plasma within theplasma chamber;

(d) supplying the plasma to an inlet end of the first grid so that theplasma passes through the first grid towards an outlet end thereof;

(e) accelerating the plasma between the outlet end of the first grid andan inlet end of the second grid so that the plasma passes through thesecond grid towards an outlet end thereof;

(f) further accelerating the plasma between the outlet end of the secondpositive grid and an inlet end of the third grid so that the plasmapasses through the third grid towards an outlet end thereof; and

(g) recovering an ion beam from the outlet end of the third grid.

The invention further provides a low energy ion beam processingapparatus comprising

(1) a vacuum chamber;

(2) an ion gun arranged to project an ion beam into the vacuum chamber;

(3) an ion beam neutraliser for projecting electrons into the ion beam;and

(4) a support for a target or a substrate in the path of the ion beam;the ion gun comprising:

a plasma chamber comprising an open ended, conductive, non-magneticbody, a first end of which is closed by a flat or minimally disheddielectric member and with electrodes at a second end thereof oppositethe first end;

primary magnet means arranged around the body for trapping electronsadjacent the wall of the plasma chamber in use of the ion gun; and

an r.f. induction device including a substantially flat coil which liesadjacent to the dielectric member for inductively generating a plasma inthe plasma chamber, characterised in that the electrodes include a firstmulti-aperture grid arranged for connection to a first positivepotential source and positioned to contact the plasma in the plasmachamber;

a second multi-aperture grid arranged for connection to a secondpotential source of lower potential than the first source so as toproduce a first acceleration field for accelerating ions towards andthrough the second grid; and

a third multi-aperture grid arranged for connection to earth or to anegative potential source so as to produce a second acceleration fieldfor accelerating ions towards and through the third grid, the aperturesof the first, second and third grids being aligned so that particlesemerging from an aperture of the first grid are accelerated through acorresponding aperture of the second grid and then through acorresponding aperture of the third grid in the form of a beamlet, aplurality of beamlets from the third grid forming a beam downstream ofthe third grid.

By “low energy” is meant up to about 10 kV, for example 5 kV or less.Usually, the ion beam generated by the apparatus of the invention willhave an energy of 1 kV or less.

The ion gun of the invention is capable of generating an ion beam ofsignificantly lower divergence than has conventionally been achievable.A 500 eV ion beam generated by a gun according to the invention may havea divergence of as little as 1°. This compares directly with values ofbetween 3° and 5° for prior art ion beams utilising conventional threegrid electrode grid structures. It has surprisingly been discovered thatan underlying design rule for ultra-low divergence ion beams has notbeen recognised in the prior art. The basis of the prior art, asexemplified by EP-B-0462165, lies in the electrostatic lens principleunderpinning the simple two/three grid conventional acceleratorstructures and its balance with the natural space charge repulsive forcein the beam. This repulsive force leads to an irreducible divergencelimit for such structures. The ion beams of the prior art are vigorouslycompressed by a strong accelerating force provided by a first,positively charged grid and a second, negatively charged grid. Thepotential difference between the first and second grids may be of theorder of 1000 V. As the beam passes through the second grid, the spacecharge force reaches a maximum and acts upon the beam to cause it todiverge as it propagates beyond the second grid. The space charge forceincreases with increasing beam current, with reducing beam radius andwith reducing beam energy. Empirically it has been found that the lowerlimit of divergence for a 500 eV beam for a three grid accelerator witha beam current viable for industrial processing lies between 3° and 5°.In contrast, the ion gun of the present invention is capable of yieldinga divergence value of 1°.

One preferred way in which the angular divergence of the beam may beminimised in the present invention is by adopting a more gentleacceleration field between the first and second grids than between thesecond and third grids. This allows the beamlets to propagate at alarger net area, hence reducing the space charge repulsion inside theaccelerator grid structure itself.

The ion gun of the invention may of course be provided with more thanthree grids. For example, three, four or more positive biassed grids,each of successively lower positive bias than its upstream neighbour,could be used. Alternatively, or in addition, a plurality of negativelybiassed or earthed grids could be incorporated towards the downstreamend of the grid structure. However, for most applications, it isenvisaged that a three or four grid structure will be preferred. In thethree grid structure, the first and second grids may be positivelybiassed, the first grid being in contact with the plasma in the plasmachamber, the second grid being of lower positive bias than the firstgrid, and the third grid may be negatively biassed or earthed. In thefour grid structure the first and second grids may be positivelybiassed, as described above, while the third grid may be negativelybiassed and the fourth grid is earthed or negatively biassed. Thus, inone preferred embodiment of the invention, the third grid of theelectrodes is arranged for connection to a negative potential source andthe electrodes include a fourth grid arranged for connection to earth.The fourth grid may be used to provide an extra degree of control overthe rates of acceleration and divergence of the ion beam.

Preferably, the grid arrangement is rigid since the mechanicalseparation of the grids plays a large part in determining the divergenceof the beam. For example, a variation of 10% or more in the distancebetween two grids can have a significant impact on the net divergence ofa large area beam. Furtherrnore, the relationship between beamdivergence and the magnitude of the gap between the grids issubstantially non-linear.

However, beam divergence is also a function of the local ion currentdensity in the beam. As the cross-sectional area of the beam increases,the current density in the beam cross-section may vary by up to about10%. The current density is lower towards the periphery of the beam.

In one embodiment of the invention, the grids are arranged in parallelalignment with each other. The gap between neighbouring grids ispreferably between about 0.5 mm and about 3.0 mm, typically about 1.00mm.

In a preferred embodiment, however, one grid of a neighbouring pair ofgrids is contoured so that the gap between the two grids of the pair islarger towards the periphery of the grids than towards the centre of thegrids. For example, the gap at the centre of the pair may be about 1.00mm while the gap at the periphery is about 1.3 mm. Preferably, thesecond grid is contoured.

The variation in current density across the beam is usually constant andrepeatable and this may be exploited to obtain best average divergenceacross a large beam. Numerical simulation may be used to confirm thisvariation with respect to the magnitude of the in gap betweenneighbouring grids and the current density in the beam.

In one preferred embodiment of the invention, four grids are provided,the gap between each grid, at the centre thereof, being about 1.00 mm.Preferably, one or more of the grids is contoured as described above.Even more preferably, only the second grid is contoured.

The ion gun of the invention is of particular value for generating lowenergy beams of heavy ions, such as argon, which are commonly used inion milling applications. Since the space charge force increases ininverse proportion to the ion velocity, the effect on the divergence ofan argon ion beam at 500 eV is over 50 times larger than for a hydrogenion beam at 50 KeV for a comparable beam current. Other heavy ionscommonly used in ion milling applications include ions derived fromkrypton, xenon, H₂, O₂, Cl₂, N₂, CO₂, SF₆, C₂F₆ or a C₂F₆/CHF₃ mixture.

In the ion gun of the present invention inductive r.f. coupling is usedto generate a plasma in the plasma chamber. The resulting plasmatypically exhibits a plasma potential that is no more than a few tens ofvolts above the potential of the plasma chamber or of the highestpotential of the internal surface thereof. This is in contrast to manyof the prior art designs of ion gun which utilise capacitative r.f.coupling to generate the plasma and which form a plasma with a plasmapotential of some hundreds of volts.

The wall means may be constructed from an electrically conductivematerial. However, if it is desired, for example, to avoid anypossibility of contamination of the ion beam by metallic ioncontaminants, then the wall means may be constructed from a dielectricmaterial.

The primary magnet means may comprise an array of magnets arranged toproduce lines of magnetic flux within the plasma chamber which extend ina curve from the wall of the plasma chamber and return thereto so as toform an arch over a respective one of a plurality of wall regions ofsaid plasma chamber, for example, wall regions which extendsubstantially longitudinally of the wall of the plasma chamber. Rareearth magnets are preferably used. Specific arrangements of the primarymagnet means are described in EP-B-0462165 and are well understood bythose skilled in the art.

It is preferred to use as near flat dielectric member as possible. Henceminimal dishing of the dielectric member is preferred. However, it maynot be practical to avoid all dishing of the dielectric member as itmust be ensured that the integrity of the vacuum equipment be preservedand that all risk of fracture of the dielectric member due to pressuredifferences exerted across it during operation is substantiallyobviated.

The r.f. emitter means associated with the dielectric member comprises asubstantially flat spirally wound coil which preferably lies adjacentto, or is embedded within, the dielectric member. Hence the coil ispreferably flat or as near flat as practicable. Such a coil may take theform of a tube of conductive material e.g. copper, through which acoolant, such as water, can be passed. This type of coil, and theadvantages thereof, are also described in EP-B-0462165.

Typically the r.f. emitter means associated with the dielectric memberis arranged to be connected to an r.f. power source which operates at afrequency in the range of from about 1 MHz up to about 45 MHz, e.g. atabout 2 MHz or, more preferably, at one of the industrially allottedwavebands within this range of frequencies e.g. at 13.56 MHz or 27.12MHz or 40.68 MHz.

By appropriate choice of geometry for the spiral driving coil and bymodifying the magnetic field strength and/or distribution within theplasma chamber it is possible to tune the excitation of the dischargefor a variety of gases, e.g. Ar, O₂ or N₂.

In a preferred form an ion gun according to the invention furtherincludes secondary magnet means associated with the r.f. emitter meansfor producing a magnetic dipole field that penetrates the r.f.energising coil or other form of r.f. emitter means.

It is also possible to provide a further magnet means, hereinaftercalled a tertiary magnet means, for superimposing a longer range axialfield on top of the field produced by the multipole array of saidprimary magnet means. Such a tertiary magnet means can, for example,take the form of an electromagnet surrounding the plasma chamber whoseaxis is arranged to be substantially aligned with or parallel to that ofthe plasma chamber.

In an ion beam apparatus according to the invention it is preferred toutilise an ion beam neutraliser that is powered by an r.f. energy sourceto produce a beam of electrons that can be projected into the ion beam.Conveniently, such an r.f. energy source operates at the same frequencyas that of the r.f. generator means of the ion gun.

The invention thus may utilise an ion beam neutraliser comprising anopen ended plasma source chamber, means for admitting a plasma forminggas to the plasma source chamber, an r.f. generating coil surroundingthe plasma source chamber for generating a plasma therein, and anextraction grid structure across the open end of the plasma sourcechamber including a first grid arranged for connection to a negativepotential source and a second grid arranged for connection to a positivepotential source so as to produce a first acceleration field foraccelerating electrons towards and through the second grid of theextraction grid structure. Such an ion beam neutraliser may use an inertgas, a reactive gas or a mixture of an inert gas and a reactive gas, asplasma forming gas.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood and readilycarried into effect, some preferred forms of ion beam processingapparatus will now be described, by way of example only, with referenceto the accompanying semi-diagrammatic drawings, in which:

FIG. 1 is a vertical section through an ion beam processing apparatus;

FIG. 2 is a plan view of the top of the ion gun of the apparatus of FIG.1;

FIG. 3 is a partial horizontal section through the plasma chamber of theapparatus of FIGS. 1 and 2;

FIG. 4 is an enlarged view of part of FIG. 3;

FIG. 5 is a view of the primary magnet array of the apparatus of FIGS. 1to 4;

FIG. 6 is a vertical section on an enlarged scale through the controlgrid structure of the apparatus of FIGS. 1 to 5;

FIG. 7 illustrates the magnetic field produced by the secondary magnetsof the ion gun shown in FIGS. 1 and 2;

FIG. 8 is a vertical section through a second form of ion gunconstructed according to the invention;

FIG. 9 is a top plan view of the ion gun of FIG. 8;

FIGS. 10 and 11 are sections on the lines A—A and B—B respectively ofFIG. 9;

FIGS. 12 and 13 are a partly cut away side view and a top plan viewrespectively of the body of the ion gun of FIG. 8;

FIGS. 14 and 15 are sections on the lines C—C and D—D respectively ofFIG. 12;

FIG. 16 is an enlarged section of part of the body of the ion gun ofFIG. 8;

FIG. 17 is a schematic diagram of the axis of the tube from which ther.f. emitter coil is formed;

FIGS. 18 and 19 are respectively a section and a side view of the r.f.emitter coil;

FIG. 20 shows, for comparative purposes, a representation of ion beamdivergence in a prior art ion gun having a conventional three gridelectrode structure;

FIG. 21 shows a representation of ion beam divergence, directlycomparable to the representation shown in FIG. 20, in a four gridelectrode ion gun according to the invention;

FIG. 22 is a top plan view of an electrode grid for use in the ion gunof the invention; and

FIG. 23 is a vertical section through a third form of ion gunconstructed according to the invention, showing an electrode gridarrangement in which the second grid is shaped.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 to 7 of the drawings, an ion beam processingapparatus 1 comprises a vacuum chamber (indicated diagrammatically at 2)surmounted by an ion gun 3. Ion gun 3 comprises a plasma generator 4mounted on top of an open ended plasma chamber 5, the lower end of whichis closed by a control grid structure 6. Control grid structure 6 isdescribed in detail below. A plasma neutraliser 7 is mounted withinvacuum chamber 2 for neutralising the ion beam 8 which issues from thelower end of ion gun 3. A target 9 is placed in the path of the ion beam8.

Plasma generator 4 comprises a dielectric member 10 which closes the topopen end of plasma chamber 5. A number of gas inlet nozzles areprovided, as indicated by arrows 11, through which a plasma forming gas,such as argon, or a mixture of a plasma forming gas and a reactive gas,such as oxygen, can be admitted to the plasma chamber 5. An r.f. coil 12surmounts member 10 and is connected to a suitable r.f. power sourceoperating at, for example 13.56 MHz. Magnets 13 and 14 are provided fora purpose which will be further described below.

Plasma chamber 5 comprises an open-ended metallic body 15, made ofaluminium or of an aluminium alloy or another conductive non-magneticmaterial, within which are mounted a plurality of primary bar magnets16. For ease of assembly body 15 is made in two parts, i.e. an innerpart 17 and an outer part 18, between which the primary magnets 16 arepositioned.

As can be seen from FIG. 3, there are thirty-two primary bar magnets 16secured longitudinally to the cylindrical outer face of inner part 17.Preferably the strongest available magnets, e.g. rare earth magnets suchas samarium-cobalt magnets, are used. Typically such magnets exhibit afield strength of the order of 1 to 2 kGauss. As illustrated in FIG. 3there are thirty-two primary magnets 16. However, a larger or smallernumber of primary magnets, for example thirty or less (e.g. twenty-four)or up to forty or more (e.g. forty-eight), may be used, provided alwaysthat there is an even number of primary magnets 16. Such primary magnets16 are evenly spaced around the outer periphery of inner part 17 withtheir longest dimension arranged substantially parallel to the axis ofthe plasma chamber 5. As indicated in FIG. 4, however, the magnetic axesof primary magnets 16 are arranged radially with respect to plasmachamber 5 so that their respective north and south poles (indicated as Nand S respectively in FIG. 4) are separated in the direction of theirshortest dimension, the primary magnets 16 being arranged withalternating magnetic polarity around the periphery of inner part 17.

Above primary magnets 16 is an annular groove 19 and below them acorresponding annular groove 20. Grooves 19 and 20 communicate one withanother via spaces 21 between adjacent primary magnets 16. The grooves19 and and spaces 21 form channels for coolant fluid (e.g. water) bymeans of which the primary magnets 16 and body 15 can be cooled in use.Reference numerals 22 and 22 a indicate coolant fluid supply andwithdrawal conduits provided in annular member 23. Baffles 24, 25 areprovided in grooves 19, 20, as can be seen in FIG. 5, in order to makethe coolant fluid follow a predetermined path.

FIG. 4 indicates the lines of magnetic force 26 produced by primarymagnets 16. These lines of force extend from the inner surface of body15 into cavity 27 in plasma chamber 5 and back into the wall of cavity27 in an arch over regions 28 which extend parallel to the axis of body5.

Reverting to FIG. 1, the lower end of plasma chamber 5 is closed by acontrol grid structure 6 which is shown in more detail in FIG. 6 on agreatly enlarged scale. Grid structure 6 comprises three grids 29, 29 aand 30, each formed with aligned holes 31, 31 a and 32. Grid 29 ispositively biased while grid is negatively biased or earthed. Grid 29 ais positively biased, but more weakly so than grid 29 so as to set uptwo acceleration fields between grids 29 and 29 a and grids 29 a and 30respectively to accelerate ions towards and through grids 29 a and 30.Such grids can be manufactured from molybdenum or a molybdenum alloy orfrom carbon sheet or from a suitable aluminium alloy. Aluminium mayoffer specific benefits where heavy metal contamination is to beavoided. Typically grid 29 has a positive potential of up to about 1000V applied to it, whilst a negative potential of from 0 to about 2000 Vis applied to grid 30. Grid 29 a has a positive potential of up to about850 V or more, for example 750 applied to it. The ion beam generatedthrough this grid arrangement is discussed more fully below.

Turning now to ion beam neutraliser 7, a gas, e.g. argon or oxygen, issupplied as indicated by arrow 33 through line 34 into a hollowinsulated electrode assembly 35. The open end of electrode assembly 35is closed by a pair of grids 36 and 37. An r.f. generator coil 38surrounds electrode assembly 35. Conveniently this is driven at the samefrequency as r.f. generator coil 12, e.g. 13.56 Mfz. Grid 36 isnegatively biased, while grid 37 is positively biased so as to set up anacceleration field between grids 36 and 37 to accelerate electronstowards and through grid 37.

FIG. 2 illustrates in plan view the positions of the optional secondarymagnets 13 and 14 relative to the r.f. generator coil 12. Thesesecondary magnets produce a magnetic dipole field which penetrates theenergizing coil 12. The shape of this magnetic field is showndiagrammatically in FIG. 7. As can be seen from FIG. 7 magnets 13 and 14have their axes of magnetisation arranged so that either a north pole ora south pole faces the dielectric member 10 and so that the lines offorce 47 penetrate the r.f. generator coil 12 and form an arch over theinner face of dielectric member 10.

Reference numeral 48 indicates an r.f. power source connected to coil12; it can also be connected to coil 38. Alternatively coil 38 can haveits own separate r.f. power source. Lines 49 and 50 indicate positivesupply leads for electrode 17 and grid 29 respectively. Convenientlyelectrode 17 and grid 29 are at the same positive potential. Referencenumeral 51 indicates a negative supply lead for providing the negativebias potential on grid 30.

Vacuum chamber 2 can be evacuated by means of a suitable vacuum pumpingsystem 52 connected via line 53 to vacuum chamber 2.

In use of ion beam processing apparatus 1 vacuum chamber 2 is evacuatedto a pressure of typically about 10⁻⁵ millibar to about 10⁻⁷ Pa (1⁻⁷millibar). A plasma forming gas, e.g. argon, a reactive gas, or amixture of a plasma forming gas and a reactive gas, e.g. O₂, CO₂, Cl₂,SF₆, C₂F₆ or a C₂F₆/CHF₃ mixture is admitted via inlets 11. R.f. coil 12is then excited to generate a plasma. This plasma has a plasma potentialof at most a few tens of volts, for example about +10 V (volts) abovethe potential of the grid 29, which is normally at the same potential asthe electrode 17. Electrons released are trapped within regions 28 bythe magnetic lines of force 26. Grid 29 is biased to a positivepotential of about 900 V, while grid 29 a is biased to a positivepotential of 725 V and grid 30 is biased to a negative potential ofabout −100 V. The ions in the cavity 28 are accelerated towards and passthrough grid 29 and are then gently accelerated by the electric fieldbetween grid 29 and grid 29 a before being further accelerated by theelectric field between grid 29 a and grid 30 to emerge in the form of acollimated ion beam 8 of defined energy. After passage through grid 30ions in flight to the target 9, which is earthed, move in a deceleratingfield. The ions arrive at the earthed target 9 with an energy equal orapproximately equal to the potential of the first, positive grid 29 plusthe sheath potential. Thus, if a bias of +900 V is applied to grid 29,which is immersed in the plasma, then the ions will arrive at the target9 with a potential of about 910 V, irrespective of how high a negativepotential is applied to the third grid 30. Grid 29 a is shaped to give acontrolled variation of separation over the area of the grid. Thisenables changes in plasma density to be accommodated with minimal effecton the local beam divergence, as discussed below with reference to FIG.23.

A plasma forming gas, such as argon, a reactive gas, or a mixture of aplasma forming gas and a reactive gas, is admitted to neutraliser 7through tube 34 at a rate of from about 1 cm³ per minute to about 5 cm³per minute, usually at the higher end of this range. The r.f. generatorcoil 38 is turned on to initiate electron discharge. Once electrondischarge has started it can be maintained by use of a keeper potentialof from about 20 V to about 40 V following reduction of the gas flowrate in tube 33 to about 1 cm³ per minute.

Under the influence of the r.f. signal from coil 12 the gas supplied viainlets 11 is dissociated to form a plasma of ions and free electrons inplasma chamber 5, the ions filling the central part of chamber 5 whilstthe electrons are trapped adjacent the walls of chamber 5 by themagnetic lines of force 26. Because of the geometrical separation of ther.f. generating coil 12 from the zones 28 of the magnetic confinementregion in plasma chamber 5 the plasma in the centre part of chamber 5 issubsequently uniform and has a relatively low plasma potential. Thismeans in turn that a relatively low acceleration potential only isneeded to extract ions from this plasma and to accelerate them towardsand through control grid structure 6. Hence the risk of overheating ofor damage to control grid structure 6, and particularly grid 29, isminimised.

Neutraliser 7 delivers a stream of electrons into the path of ion beam 8and provides current neutralisation at the target 9.

As a heated cathode is not used to generate the plasma the illustratedapparatus can be used with any type of inert or reactive gas. Typicalgases that can be used include argon, krypton, xenon, H₂, O₂, Cl₂, SF₆,CO₂, CF₄, C₂F₆, CHF₃ and mixtures of two or more thereof.

As illustrated the ion beam processing apparatus 1 is set up for ionbeam milling. It is a simple matter to modify the apparatus for sputterdeposition; in this case a target replaces substrate 9 and is arrangedso that it is struck by ion beam 8 at an oblique angle, while a targetis placed in the path of the ensuing sputtered material but out of thepath of the ion beam.

FIG. 8 is a vertical section through another form of ion gun 100constructed in accordance with the invention. This comprises a body 101made of austenitic stainless steel around which are mounted twenty barmagnets 102, symmetrically disposed about the periphery of ion gun 100.(There are few magnets in this embodiment than in that of FIGS. 1 to 7because the diameter of body 101 is smaller than that of the ion gun ofFIGS. 1 to 7). Typically the magnets 102 are rare earth magnets, such assamarium-cobalt magnets, with a field strength in the range of fromabout 0,1 T to about 0,2 T (1 kilogauss to about 2 kilogauss). As can beseen from FIG. 8, the magnetic axis of each magnet 102 is aligned so asto lie radially with respect to the axis of the ion gun 100 and tocorrespond to the shortest dimension of the magnet 102. The magnets 102are arranged with alternating polarity around the periphery of ion gun100 so that the magnets adjacent to the magnet shown in FIG. 8 havetheir north poles (not their south poles) facing towards the axis of iongun 100, the next adjacent magnets to these have their south polesfacing the axis of ion gun 100, and so on. A pole piece 103 made of softiron or of a soft magnetic material surrounds body 101 and magnets 102.

Body 101 has an open upper end which is closed by dielectric end plate104 made of alumina. Alternatively it can be made of another dielectricmaterial such as silica.

Above end plate 104 is an r.f. generator coil 105 in the form of aspirally wound copper tube having four complete turns. (For the sake ofclarity the coils of r.f. generator coil are omitted in FIG. 9; however,the construction of coil 105 is shown in more detail in FIGS. 17 to 19as described (further below). Water can be pumped through coil 105 tocool it. The end portions of coil 105 are indicated by means ofreference numerals 106 and 107.

A top ring 108 holds end plate 104 in position and further providessupport for a clamp 109 for coil 105. Top ring 108 is held in place bymeans of socket headed bolts 110. An O-ring 111 serves to provide a sealbetween end plate 104 and top ring 108.

Ion gun 100 is mounted in a vacuum chamber, similar to vacuum chamber 2of the apparatus of FIGS. 1 to 8, which is designated by referencenumeral 112 and is generally similar to vacuum chamber 2. In particularit is provided with connections to a vacuum pump (not shown) and isfitted with an ion beam neutraliser (not shown) and with a target (alsono shown). The gun 100 is mounted to the body of the vacuum chamber 112by means of a spacer 113, a clamp 114, and bolts 115. An O-ring 116serves to provide a vacuum seal between pole piece 103 and spacer 113.

FIGS. 12 to 15 show the construction of body 101 in more detail. Thishas twenty slots 117 formed in its outer surface, each adapted toreceive a corresponding bar magnet 102. At the lower end of body 101there are machined a number of short grooves 118, five in all, which areevenly spaced around the periphery of body 101. There are also fourgrooves 119 at the upper end of body 101 but these are offset withrespect to grooves 118. Vertical bores 120 connect grooves 118 and 119.Bores 121 and 122 provide an inlet and outlet passage to the tortuouspath provided by grooves 118 and 119 and bores 120. As can be seen fromFIG. 8 grooves 118 are closed by means of split ring 123 which is weldedon the lower end of body 101, whilst insert plates 124 (shown in FIG.13) are welded into the upper end of body 101 to close off grooves 119.In this way a closed passage for a coolant, such as water, is formedthrough body 101 which follows a tortuous path passing between adjacentpairs of magnets 102. Inlet and outlet connections 125 and 126 areprovided to communicate with bores 121 and 122.

Body 101 carries at its lower end a control grid structure whichincludes a first grid 127, a second grid 128 and a third grid 129. Grid127 is bolted directly to body 101 and is in electrical contacttherewith. Grids 128 and 129 are supported at three points around theperiphery of body 101 by suitable insulator supports 129 a (only one ofwhich is shown). As is shown in FIG. 10, they are connected to terminals130 and 130 a by respective leads 131 and 131 a which pass throughrespective insulating pillars 132 and 132 a and then through respectiveinsulating tubes 133 and 133 a mounted in bores 134 and 134 a and whichare electrically connected to respective grids 128 and 129 by means of arespective feedthrough 135 or 135 a which passes through a respectivespacer 136 or 136 a positioned in a hole or holes in grid 127. Nut 137or 137 a and washer 138 or 138 a complete the connection to respectivegrids 128 and 129. With this arrangement grid 127 adopts the potentialof body 101, whilst grids 128 and 129 can be independently biased byapplying a suitable potential to respective terminal 130 or 130 a.

As can be seen from FIGS. 8, 10, 11 and 12 body 101 has a groove 139positioned under the edge of dielectric member 104. This communicates bymeans of a transverse oblique bore 140 (see FIG. 16) with a further bore141. Reference numeral 142 denotes a plug closing the outer end of bore140. A connection 143 for a plasma gas supply (shown in FIG. 9) isscrewed into bore 140. Such plasma gas can enter the plasma chamber 144within body 101 by leakage from groove 139 through a clearance gap 145(which is more clearly seen in FIG. 16) under dielectric member 104.

Reference numeral 146 in FIGS. 8 and 9 denotes a terminal by means ofwhich a suitable potential, usually a positive potential, can be appliedto body 101 and hence to grid 127.

FIGS. 17 and 19 illustrate the construction of spiral r.f. coil 105 ingreater details. Only the axis of the tube of coil 105 is depicted inFIG. 17. The coil has 4 complete turns between portions 106 and 107.

In use of the ion gun of FIGS. 8 to 19 water is passed through coil 105and through the tortuous path in body 101 by means of inlet 125 andoutlet 126 and vacuum chamber 112 is evacuated to a suitably lowpressure, e.g. 10⁻³ Pa to 10⁻⁵ Pa (10⁻¹ millibar to 10⁻⁷ millibar). Body101 is biased to a suitable potential, e.g. +900 V, whilst grid 128 isbiased to, for example +725V, and grid 129 to −100 V. Plasma forminggas, such as argon or a mixture of argon and a reactive gas (e.g.oxygen), is then bled into vacuum chamber 112 via inlet 43 whilstmaintaining a pressure in the range of from about 10⁻¹ Pa to about 10⁻²Pa (from about 10⁻³ millibar to about 10⁻⁴ millibar). Upon applicationof a suitable r.f. frequency, e.g. 13.56 MHz, to coil 105, a plasma isgenerated in plasma chamber 144. This typically equilibrates at a plasmapotential of about 10 volts above that of body 101 and grid 127. Ionsmigrate by thermal diffusion, it is thought, to the vicinity of grid 127and, upon passing through grid 127 are accelerated gently towards andthrough grid 128 by the electrical field caused by the potentialdifference between the two grids 127 and 128 (e.g. about 175 volts).After passage through grid 128 the ions are more rapidly accelerated(through a potential difference of about 825 volts) towards and throughgrid 129. After passing through grid 129 the ions travel on in vacuumchamber 112 towards a target (similar to target 9 of FIG. 1, but notshown) which is typically earthed. An ion beam neutraliser (not shown)similar to neutraliser 7 of FIG. 1 may be located within vacuum chamber112. Grid 129 acts further to produce a deceleration field through whichions that have traversed grid 129 have to pass before hitting thetarget. In this way an ion beam with a suitable beam potential, which isnormally approximately the same potential as that of the body 101 in thearrangement described, is produced.

Referring to FIG. 20, there can be seen the effect on ion beamdivergence of a prior art three grid system in accordance withEP-B-0462165. The ion beam indicated by reference numeral 147 isaccelerated from the plasma mass 148 towards grid 29(p) which is held ata positive potential of around 500 V. Rapid acceleration then occursthrough the electric field indicated by field lines 149 which focus theion beam into a divergent beam through grid 30(p) which is held at apotential of around −500 V. Grid 31(p) is connected to earth. Thecolossal acceleration of the ion beam through the 1000 V potentialdifference between grids 29(p) and 30(p) gives rise to a very strongspace charge repulsive force in the region of grid 30(p). This repulsiveforce causes the strong divergence, seen at 150, of around 79.3 mrad(i.e. about 4°).

In contrast, FIG. 21 shows the effect of the electrode grid arrangementaccording to the invention. First grid 29 is held at a positivepotential of 900 V whilst second grid 29 a is held at a lower positivepotential of 725 V. Second grid 29 a is contoured as shown in FIG. 23 sothat the grid spacing between the first and second grids is greatertowards the periphery of the grids than it is towards the centrethereof. The ion acceleration between grids 29 and 29 a is gentler (apotential difference of just 175 V) than in the prior art grid of FIG.20. This allows formation of a more stably collimated beam which is lesssusceptible to space charge repulsive forces in the region of third grid30 and hence shows a divergence of just 15.7 mrad (less than 1°). In theembodiment shown in FIG. 21, four electrode grids are used. Third grid30 is held at a negative potential of −100 V whilst fourth grid 30 a isconnected to earth.

FIG. 22 shows a suitable grid in which holes 31, 31 a, or 32 can beseen.

FIG. 23 shows more clearly the preferred form of grid arrangementaccording to the invention, in which the second grid 29 a; 128 iscontoured. The ion beams passing through individual holes will repeleach other to a certain extent. If the middle grid is arcuate in sectionit will tend to focus the ion beams to a point but then the repulsionwill tend to make them straight and parallel to each other.

It will be appreciated by those skilled in the art that the illustratedion guns can be used in inert gas ion beam etching; in reactive ion beametching, or in chemically assisted ion beam etching by suitable choiceof the gas or gases supplied to the plasma chamber and to the ion beamneutraliser.

What is claimed is:
 1. Apparatus for the production of low energycharged article beams comprising: a plasma chamber; means for generatingin the plasma chamber a plasma comprising particles of a first polarityand oppositely charged particles of a second polarity; means forrestraining particles of the first polarity in the plasma chamber; afirst multi-aperture electrode grid contacting the plasma, wherein thefirst electrode grid is arranged for connection to a first potentialsource so as to impart to the first electrode grid a first potential ofthe second polarity; a second multi-aperture electrode grid arranged forconnection to a second potential source so as to impart to the secondelectrode grid a second potential, the second potential being less thanthe first potential so as to produce between the first and secondelectrode grids a first acceleration field for accelerating chargedparticles of the second polarity towards and through the second grid;and a third multiaperture electrode grid arranged for connection to athird potential source so as to impart to the third electrode grid athird potential of the first polarity and to produce between the secondand third electrode grids a second acceleration field for acceleratingcharged particles of the second polarity towards and through the thirdelectrode grid, the grid spacing between the first and second gridsbeing greater at the periphery of the grids than at the centre thereof,the apertures of the first, second and third grids being aligned so thatparticles emerging from an aperture of the first grid are acceleratedthrough a corresponding aperture of the second grid and then through acorresponding aperture of the third grid in the form of a beamlet, aplurality of beamlets from the third grid forming a beam downstream ofthe third grid.
 2. Apparatus according to claim 1, wherein the chargedparticles of the first polarity are ions and the charged particles ofthe second polarity are electrons.
 3. Apparatus according to claim 1,wherein the charged particles of the first polarity are electrons andthe charged particles of the second polarity are ions.
 4. A low energyion gun for use in ion beam processing comprising: a plasma chambercomprising an open ended, conductive, non-magnetic body, a first end ofwhich is closed by a flat or minimally dished dielectric member and withelectrodes at a second end thereof opposite the first end; primarymagnet means arranged around the body for trapping electrons adjacentthe wall of the plasma chamber in use of the ion gun; and an r.f.induction device including a substantially flat coil which lies adjacentto the dielectric member for inductively generating a plasma in theplasma chamber, said electrodes including a first multi-aperture gridarranged for connection to a first positive potential source andpositioned to contact the plasma in the plasma chamber; a secondmulti-aperture grid arranged for connection to a second potential sourceof lower potential than the first source so as to produce a firstacceleration field for accelerating ions towards and through the secondgrid; and a third multi-aperture grid arranged for connection to a thirdpotential source of lower potential than the second potential source soas to produce a second acceleration field for accelerating ions towardsand through the third grid, the grid spacing between the first andsecond grids being greater at the periphery of the grids than at thecentre thereof, the apertures of the first, second and third grids beingaligned so that particles emerging from an aperture of the first gridare accelerated through a corresponding aperture of the second grid andthen through a corresponding aperture of the third grid in the form of abeamlet, a plurality of beamlets from the third grid forming a beamdownstream of the third grid.
 5. An ion gun according to claim 4,wherein the third potential source is arranged to impart a negativepotential to the third grid.
 6. An ion gun according to claim 4, whereinthe third potential source is arranged to earth the third grid.
 7. Anion gun according to claim 6, wherein a fourth grid is provided and isarranged for connection to earth.
 8. A low energy ion gun for use in ionbeam processing comprising: a plasma chamber comprising an open ended,conductive, non-magnetic body, a first end of which is closed by a flator minimally dished dielectric member and with electrodes at a secondend thereof opposite the first end; primary magnet means arranged aroundthe body for trapping electrons adjacent the wall of the plasma chamberin use of the ion gun; and an r.f. induction device including asubstantially flat coil which lies adjacent to the dielectric member forinductively generating a plasma in the plasma chamber, said electrodesincluding a first multi-aperture grid arranged for a connection to afirst positive potential source and positioned to contact the plasma inthe plasma chamber; a second multi-aperture grid arranged for connectionto a second potential source of lower potential than the first source soas to produce a first acceleration field for accelerating ions towardsand through the second grid; a third multi-aperture grid arranged forconnection to a negative potential source so as to produce a secondacceleration field for accelerating ions towards and through the thirdgrid; and a fourth multi-aperture grid arranged for connection to earth,the apertures of the first, second, third and fourth grids being alignedso that ions emerging from an aperture of the first grid are acceleratedthrough a corresponding aperture of the second grid and then through acorresponding aperture of the third grid before passing through acorresponding aperture of the fourth grid in the form of a beamlet, aplurality of beamlets from the fourth grid forming a beam downstream ofthe fourth grid.
 9. An ion gun according to claim 8, wherein thepotential difference between the second and third grids is greater thanthe potential difference between the first and second grids so that thefirst acceleration field is more gentle than the second accelerationfield.
 10. An ion gun according to claim 8, wherein the grid spacingbetween first and second grids is greater at the periphery of the gridsthan at the centre thereof.
 11. An ion gun according to claim 10,wherein the grid spacing at the periphery of the grids is about 30%greater than the grid spacing at the centre of the grids.
 12. An ion gunaccording to claim 10, wherein at least one of the first and secondgrids is contoured to provide a decreased grid spacing towards thecentral region of the grids relative to the peripheral region thereof.13. An ion gun according to claim 8, wherein each grid comprises aregular array of apertures 2 mm to 6 mm in diameter.
 14. An ion gunaccording to claim 13, wherein each of the apertures is about 4 mm indiameter.
 15. An ion gun according to claim 8, wherein the grid spacingbetween adjacent apertures in the region of the centre of neighbouringgrids is from about 0.5 mm to 3.0 mm.
 16. An ion gun according to claim15, wherein the grid spacing between adjacent apertures in the region ofthe centre of neighbouring grids is about 1 mm.
 17. An ion gun accordingto claim 8, wherein the grids are made of a rigid material.
 18. An iongun according to claim 8, wherein the dielectric member is flat.
 19. Anion gun according to claim 8, wherein the coil lies outside and adjacentto, or is embedded within, the dielectric member.
 20. An ion gunaccording to claim 8, wherein the coil is arranged to operate at afrequency in the range of from about 1 MHz to about 40 MHz.
 21. An iongun according to claim 8, wherein said primary magnet means comprises anarray of magnets arranged to produce lines of magnetic flux within theplasma chamber which extend in a curve from the wall of the plasmachamber and return thereto so as to form an arch over a respective oneof a plurality of wall regions of said plasma chamber.
 22. An ion gunaccording to claim 21, wherein said wall regions extend longitudinallyof the wall of said plasma chamber from one end of the chamber toanother.
 23. An ion gun according to claim 8, wherein said primarymagnet means comprises an even number of magnets arranged in an arrayincluding at least one row extending around the periphery of said plasmachamber, each magnet in said array being disposed with its magnetic axisextending substantially in a lateral plane and having a pole of oppositepolarity to that of the magnets adjacent to it facing towards the plasmachamber.
 24. An ion gun according to claim 23, wherein the array ofmagnets comprises a plurality of rows of magnets extending around theperiphery of said plasma chamber with each magnet in a row having a poleof opposite polarity to that of any adjacent magnet in another adjacentrow facing towards the plasma chamber.
 25. An ion gun according to claim8, further comprising secondary magnet means associated with the r.f.emitter coil for producing a magnetic dipole field that penetrates ther.f. emitter coil.
 26. A method for generating a low energy ion beamcomprising the steps of: (a) providing an ion gun comprising: a plasmachamber having an open ended, conductive, non-magnetic body, a first endof which is closed by a flat or minimally dished dielectric member andwith electrodes at a second end thereof opposite the first end; primarymagnet means arranged around the body for trapping electrons adjacentthe wall of the plasma chamber in use of the ion gun, and an r.f.induction device including a substantially flat coil which lies adjacentto the dielectric member for inductively generating a plasma in theplasma chamber, said electrodes including a first multi-aperture gridarranged for a connection to a first positive potential source andpositioned to contact the plasma in the plasma chamber; a secondmulti-aperture grid arranged for connection to a second potential sourceof lower potential than the first source so as to produce a firstacceleration field for accelerating ions towards and through the secondgrid; a third multi-aperture grid arranged for connection to a negativepotential source so as to produce a second acceleration field foraccelerating ions towards and through the third grid; and a fourthmulti-aperture grid arranged for connection to earth, the apertures ofthe first, second, third, and fourth grids being aligned so that ionsemerging from an aperture of the first grid are accelerated through acorresponding aperture of the second grid and then through acorresponding aperture of the third grid before passing through acorresponding aperture of the fourth grid in the form of a beamlet, aplurality of beamlets from the fourth grid forming a beam downstream ofthe fourth grid; (b) supplying to the plasma chamber a plasma forminggas; (c) exciting the r.f. induction device to generate a plasma withinthe plasma chamber; (d) supplying the plasma to an inlet end of thefirst grid so that the plasma passes through the first grid towards anoutlet end thereof; (e) accelerating the plasma between the outlet endof the first grid and an inlet end of the second grid so that the plasmapasses through the second grid towards an outlet end thereof; (f)further accelerating the plasma between the outlet end of the secondpositive grid and an inlet end of the third grid so that the plasmapasses through the third grid towards an outlet end thereof; and (g)recovering a plurality of ion beamlets from the outlet end of the thirdgrid.
 27. A low energy ion beam processing apparatus comprising: (1) avacuum chamber; (2) an ion gun arranged to project an ion beam into thevacuum chamber; (3) an ion beam neutraliser for projecting electronsinto the ion beam; and (4) a support for a target or a substrate in thepath of the ion beam, wherein the ion gun comprises: a plasma chamberhaving an open ended, conductive, non-magnetic body, a first end ofwhich is closed by a flat or minimally dished dielectric member and withelectrodes at a second end thereof opposite the first end; primarymagnet means arranged around the body for trapping electrons adjacentthe wall of the plasma chamber in use of the ion gun; and an r.f.induction device including a substantially flat coil which lies adjacentto the dielectric member for inductively generating a plasma in theplasma chamber, said electrodes including a first multi-aperture gridarranged for a connection to a first positive potential source andpositioned to contact the plasma in the plasma chamber; a secondmulti-aperture grid arranged for connection to a second potential sourceof lower potential than the first source so as to produce a firstacceleration field for accelerating ions towards and through the secondgrid; a third multi-aperture grid arranged for connection to a negativepotential source so as to produce a second acceleration field foraccelerating ions towards and through the third grid; and a fourthmultiaperture grid arranged for connection to earth, the apertures ofthe first, second, third, and fourth grids being aligned so that ionsemerging from an aperture of the first grid are accelerated through acorresponding aperture of the second grid and then through acorresponding aperture of the third grid before passing through acorresponding aperture of the fourth grid in the form of a beamlet, aplurality of beamlets from the fourth grid forming a beam downstream ofthe fourth grid.
 28. An ion beam processing apparatus according to claim27, wherein the ion beam neutraliser is powered by an r.f. energysource.
 29. An ion beam processing apparatus according to claim 27,wherein the ion bean neutraliser comprises an open ended plasma sourcechamber means for admitting a plasma forming gas to the plasma sourcechamber, an r.f. generating coil surrounding the plasma source chamberfor inductively generating a plasma therein, and an extraction gridstructure across the open end of the plasma source chamber including afirst grid arranged for connection to a potential source, and a secondgrid arranged for connection to a positive potential source so as toproduce an acceleration field for accelerating electrons towards andthrough the second grid of the extraction grid structure.